The general objective of this project is to carry out a complete morphological description of the nervous system of a simple organism during its embryological development and to study the way in which this development can be altered by genetic and environmental factors. We have developed optical and computer methods which allow thousands of electron or light micrographs of serial brain sections to be combined and analyzed in a way which permits the computer to be used as a very large three-dimensional notebook. A three-dimensional map of the brain can be constructed in the computer, and this mapping repeated at several times during the development of the embryo. These three-dimensional reconstructions have allowed us to detect several new phenomena associated with growing nerve fibers and the cells they contact. We propose to examine these phenomena and their significance in greater detail both morphologically and by experimental modification of growing embryos. In addition, we propose to look for the same type of interaction in other nervous tissue and in other organisms, including the water-flea Daphnia Magna, giant prawn Macrobrachium Rosenbergii, and the tropical fish Poecilia Formosa. The importance of the Daphnia and P. Formosa is that both reproduce parthenogenetically and therefore produce clones of genetically identical animals and we can be reasonably sure that observed variations are not due to genetic difference. In the Daphnia we can look at a small number of inter-connecting cells in the optic system and in the P. Formosa we can look at cells which are identified as to their particular type. These two animals span the range from a simple invertebrate to a relatively complex vertebrate. The Macrobrachium has the advantage that developmental neuro-anatomy can be studied on eggs and larvae, micro- lesions can be induced at these stages and the adults can be studied both anatomically and physiologically.